Inorganic layers and coatings are often formed on a surface of a substantially chromium-free metal component to protect and mask the underlying metal substrate and to extend its useful life, such as by providing corrosion resistance and chemical resistance. Although zinc may be applied as an inorganic protective coating on iron or steel substrates either electrochemically by plating or by hot dip methods, such zinc protective coatings may be subject to corrosion in the form of a white rust defect. To prevent the occurrence of white rust defects, a conversion coating such as a chromate may be applied as a pretreatment on the iron or steel substrate before the zinc is applied. However, the process forming chromate coatings presents environmental and health risks that reduce their prevalence of use and that have instigated research to identify suitable substitute environmentally friendly coatings to replace chromate coatings.
Hard chrome plating is another common inorganic protective coating used in industrial equipment to reduce friction and wear. Hard chrome coatings are applied by electrolytic deposition processes. However, electrolytic deposition processes involve chemicals that are extremely hazardous, toxic, corrosive, and damaging to the environment. The use, storage, and disposal of the chemicals involved in electrolytic deposition processes are governed by law in most jurisdictions. Personal safety equipment and proper containment facilities are considered mandatory.
Copper is most frequently used as a pure, unalloyed metal. For example, copper tubing and pipes are used to transport potable water in a residence. However, copper corrodes over time and, if sulfur has been present, turns blue due to the formation of copper sulfate from sulfur exposure.
Organic (i.e., carbon-based) coatings, such as conventional paints, may also be used to protect substantially chromium-free metal substrates. However, organic coatings may deteriorate at high temperatures because of the presence of a polymer operating as a binder to confer film-forming properties. Consequently, organic coatings may lose their protective ability such that the formerly protected metal substrate is vulnerable to corrosion or chemical attack.